Structure and dynamics of a pinned vortex liquid in a superconducting a-Re₆Zr thin film

We report the formation of a pinned vortex liquid spanning a very large region of the magnetic fieldtemperature parameter space in a 5-nm-thick amorphous superconducting RexZr (x approximate to 6) (a-ReZr) thin film, using a combination of low-temperature scanning tunneling spectroscopic (STS) imaging and magnetotransport measurements. The nature of the vortex liquid differs significantly from a regular liquid. Analyzing series of STS images captured as a function of time, we observe that the interplay of pinning and intervortex interactions produces a very inhomogeneous state, where some vortices remain static, whereas others move forming a percolating network along which vortices are mobile. With increase in temperature or magnetic field this network becomes denser, eventually encompassing all vortices. Our results provide key insight on the nature of a pinned vortex liquid and some of the peculiarities in the transport properties of ultrathin superconducting films.